For words that have only three letters, there's no 4th letter. By way of example with the word “insert”, there won't be any extra letters Later on and thus must be the word "insert". The word “addict” is from the BIP39 wordlist but you would have made use of “addi” https://myeasybookmarks.com/story4363519/facts-about-bip39-seed-phrase-word-list-revealed